In general, a photo mask is used during photolithography, and photolithography is widely adopted in the fabrication of semiconductor devices. The photo mask includes a light transmitting portion and a light shielding portion that combine to expose an underlying surface to a distribution of light. In other words, the photo mask generally includes a light transmitting pattern and a light shielding pattern such that light passing therethrough produces a selective exposure process.
However, as packing density of the pattern increases, diffraction phenomenon of light occurs, which prevents the resolution from being improved. In this regard, a process which improves the resolution using a phase shifting mask has been studied and developed in many fields.
The process using the phase shifting mask includes a light transmitting region which transmits light as it is and a light shifting region which transmits light and shifts it 180.degree. out of phase in order not to reduce the resolution between the light transmitting pattern and the light shielding pattern. In addition, with improvement of the process steps of fabricating the mask, a variety of masks using a phase difference of light have been developed, thereby increasing the limitation of the resolution. For example, there are a RIM type phase shifting mask by Nitayama, and an attenuated phase shifting mask. The RIM type phase shifting mask improves the resolution of a contact hole. The attenuated phase shifting mask reduces the area of the phase shifting mask and is referred to as a half tone phase shifting mask or t.pi. phase shifting mask. Here t means transmittance.
The conventional method for fabricating a phase shifting mask will be described with reference to the accompanying drawings.
FIG. 1a to FIG. 1h are sectional views of fabricating process steps for a conventional phase shifting mask.
As shown in FIG. 1a, a light shielding layer 11 and a first photoresist 12 are sequentially formed on a light transmitting substrate 10. The light shielding layer 11 is formed of Cr and has a predetermined light transmittance ratio, determined by a predetermined thickness sufficient to be opaque to light.
As shown in FIG. 1b, the first photoresist 12 is patterned by exposure and developing processes to define light shielding regions (lying under the remaining portions of the first photoresist 12).
As shown in FIG. 1c, the light shielding layer 11 is selectively removed by an etching process using the first photoresist 12 as a mask to form openings 3 through which the light transmitting substrate 10 is exposed to light.
As shown in FIG. 1d, the first photoresist 12 on the light shielding layer 11 is removed.
As shown in FIG. 1e, a second photoresist 13 is formed on the overall exposed surface including being formed in the openings 3.
As shown in FIG. 1f, the second photoresist 13 selectively patterned by exposure and developing processes to alternately expose one of the openings 3. The second photoresist 13 has a predetermined alignment margin on the light shielding layer 11.
As shown in FIG. 1g, the light transmitting substrate 10 under the exposed opening 3 is etched to remove enough material so that a phase shifting thickness d remains. The etching process uses the patterned second photoresist 13 as a mask so as to form a phase shifting region 4. The phase shifting thickness d can be expressed by the following formula (1). ##EQU1##
Here, .lambda. is the length of exposure wavelength, and n is the refractive index of the substrate.
In order to etch the light transmitting substrate 1 to remove an amount of the substrate material so that the phase shifting thickness d remains, an etch back process using reactive ion etching (RIE) process is performed. In this case, CF.sub.4 gas is used as the etching gas.
As shown in FIG. 1h, the portions of the second photoresist 13 remaining on the light transmitting substrate 10 and the light shielding layer 11 are finally removed.
The conventional method for fabricating the phase shifting mask has several problems.
First, when the light transmitting substrate 10 is etched to remove enough material so that the phase shifting thickness remains (using the photoresist as a mask by RIE process), a predetermined portion of the light shielding layer 11 is not masked by the photoresist 13 due to the predetermined alignment ratio. This limits the etching ratio between the light shielding layer 11 and the light transmitting substrate 10 because portions of the light shielding layer 11 will be exposed to the etchant. Therefore, because an amount of the light shielding layer is partially and undesirably etched by the CF.sub.4 gas used as an etching gas, the light transmittance ratio of the light shielding layer increases. Either the initial thickness of the light shielding layer must be increased so that a minimum thickness remains after the undesired etching or the additional transmittance near the phase shifting region must be tolerated. As a result, it is difficult to accurately pattern the light shielding layer. In addition, reliability of the phase shifting mask is reduced.
When the light shielding layer 11 is partially and undesirably etched, Cr (which forms the light shielding layer) is mixed with the etching gas CF.sub.4, thereby resulting in problems that Cr particles are formed on, or deposited in, the phase shifting region of the light transmitting substrate 10. This contamination causes the phase shifting mask to fail to accurately transfer the pattern.